Clinical trials of the radiation protector S-2-(3-aminopropylamino)ethyl phosphorothioic acid (WR-2721) with radiotherapy are currently in progress both in this country and in the Japan. WR-2721 protects most normal tissues, except the brain and the spinal cord, against the harmful effects of ionizing radiation without appreciable protection of certain tumors. This drug also protects against adverse toxic side effects of cancer chemotherapeutic agents such as mechlorethamine, cyclophosphamide and cis-dichlorammine platinum. In order to achieve selective protection of the host tissue without lowering the radiosensitivity of the tumor, the concentration of the drug in the tumor should be low compared to that in the normal tissue. Phamacokinetics based on reliable assays for WR-2721 and related compounds are needed for developing therapeutic strategies. 31 Phosphorus nuclear magnetic resonance (31P-NMR) is useful for detecting WR-2721 and its analog, sodium hydrogen S-(3-amino-2-hydroxypropyl) phosphothioate (WR-77913) in biological samples. Surface coils will be used to obtain 31P-NMR spectra of muscle and solid tumors growing in live mice. Spectra will be recorded before and at different times after administration of graded doses (up to the maximum tolerated dose) of WR-2721 and WR-77913 to tumor bearing mice in order to monitor its appearance and disappearance from the muscle and tumor in the intact animal. The KHT sarcoma, the EMT6 tumor and the SL2 leukemia growing as a solid tumor in mice will be studied. These experiments will yield information regarding the degree of hypoxia, microvasculature, the concentration of phosphorothioate and the dephosphorylation activity in the tumor compared to normal muscle. In vitro 31P-NMR will be utilized to estimate the dephosphorylation of WR-2721 by normal tissue and tumors. The dephosphorylation activity will be determined in homogenates of lung, muscle, kidney, liver and spleen as well as KHT, EMT6 and SL2 tumor cells. In vivo 31P-NMR should prove valuable for noninvasive and nondestructive monitoring of the levels of phosphothioates and other phosphorus containing pharmaceuticals in normal and diseased states.